The invention relates to a method for operating an energy storage unit having a plurality of electrically interconnected battery cells that each comprise an overvoltage protection apparatus and a cell fuse, wherein a respective battery cell voltage applied to a battery cell of the energy storage unit is monitored for whether it is greater than a prescribed cell voltage minimum value, the energy storage unit being disconnected in the event of a drop in the battery cell voltage below the cell voltage minimum value.
Furthermore, the invention relates to a battery management system for monitoring and regulating operation of an energy storage unit having a plurality of electrically interconnected battery cells.
Moreover, the invention relates to an energy storage unit having a battery management system.
Energy storage units having a plurality of electrically interconnected battery cells are of great significance, particularly for electromobility. As such, battery systems having a plurality of secondary battery cells that are electrically interconnected to increase the power and/or capacity of the battery system are used as energy storage units in hybrid, plug-in hybrid and electric vehicles. In particular, this involves a plurality of battery cells being electrically connected up to form a battery module, with multiple battery modules in turn being connected up to form a battery system.
The battery cells used in such a battery system are lithium ion cells in prismatic form, in particular. During operation of the battery cells, particularly in the case of lithium ion cells, it is necessary to ensure that they are operated within certain operating limits in order to ensure a long life for the cells and the safety of the battery system. To this end, a battery system usually comprises a battery management system that is designed to monitor and regulate operation of the battery system.
In particular, multiple safety devices are known for battery systems or battery cells that, in the event of a fault or defect occurring, contribute to no risk being posed by the battery system.
By way of example, disorders such as overcharging of the battery cell during a charging process can lead to critical heating of the battery cell, which can end in thermal runaway of the battery cell. In particular, unwanted chemical reactions then start in the battery cell. As such, the electrolyte required for ion transport between the anode and the cathode of a battery cell can break down and change to a gaseous state as a result of the critical heating.
In order to prevent such gas formation in a rechargeable battery cell as a result of overcharging of the battery cell, battery cells are known that have an overvoltage protection apparatus, also called an “overcharge safety device” (OSD). A battery cell having such an overvoltage protection apparatus is known from the documents DE 10 2011 002 659 A1 and DE 10 2012 200 868 A1, for example.
Disadvantageously, tripping of the overvoltage protection apparatus has to date immediately resulted in disconnection of the energy storage unit, particularly because the battery cell voltage of the affected cell then falls below a cell voltage minimum value and a fall in a battery cell voltage below a cell voltage minimum value is a fault state. When an energy storage unit used in an electric vehicle is disconnected, this disadvantageously results in the vehicle breaking down. It is then not possible to continue driving.
Against this background, it is an object of the invention to provide a method and means that allow improved operation of an energy storage unit having a plurality of electrically interconnected battery cells. In particular, the intention in this case is to reduce the occurrence of disconnection of an energy storage unit further, so that electric load devices, in particular hybrid, plug-in hybrid or electric vehicles, connected to the energy storage unit can advantageously be operated with additionally increased reliability.